The present invention is directed to a high temperature, zero leakage packing assembly for use in connection with the valve stem of a high pressure control valve. The invention is particularly suitable for use in the control of high temperature, high pressure fluids containing corrosive materials, such as molten salt.
In accordance with a well known design for a high pressure fluid control valve, a valve plug is selectively matable with a valve seat formed within the internal flow path of a valve body to open and close fluid flow through the valve. Typically, the valve plug is controlled to move through a predetermined work stroke by a valve stem. The valve stem extends through an opening formed in the valve body to an actuator mechanism, whereby the valve plug may be selectively controlled to operate the valve. Of course, it is necessary, in the conventional design, to provide a sealing means to form a leak tight seal between the valve stem and the valve body opening to prevent fluid leakage around the stem.
An example of a widely used sealing means consists of a plurality of ring-shaped packing elements arranged in an axial stack and received over the valve stem. The ring seals are dimensioned to be in a snug fit between the stem and inner diameter of the valve body opening through which the stem extends. In this manner, the axial stack of elements provides a leak-tight fit between the stem and body while accommodating axial displacement of the stem to control the valve plug. While the prior art arrangement is generally effective as a seal, the ring elements are subject to erosion and breakdown when the fluid passing through the valve contains corrosive materials. When, for example, the fluid is under high pressure and contains a molten salt, the high pressure effect of the fluid will constantly place the corrosive salt against the ring elements, whereby the ring elements are gradually eroded away until the elements are no longer effective as a seal. At such time, the valve must be repaired, thereby significantly interfering with the on-line work life of the valve and increasing the maintenance costs associated with the valve.
It is a primary objective of the present invention to provide a new and improved stem seal arrangement which is resistant to erosion and highly convenient to repair and/or replace with minimal valve down time. Generally, the seal of the invention comprises a plurality of sealing components arranged to provide a series of sealing barriers and to impede and minimize the exposure of the sealing barriers to the corrosive contents of the fluid flow. The sealing arrangement includes a lowermost bushing and an uppermost safety bushing arranged in an axially spaced relation with respect to one another and securing therebetween a series of packing ring elements. The bushings and ring elements are each in an annular configuration to conveniently fit in a leak tight relation between the valve stem and valve body opening. Each of the bushings is provided with fluid seal elements to form first and last fluid sealing barriers, while the series of packing elements form several intermediate fluid sealing barriers therebetween.
Pursuant to an important feature of the invention, the lowermost bushing comprises an erosion resistant material, such as stainless steel, and includes an internal fluid chamber. The bushing itself will act to substantially impede fluid leakage. Moreover, any minimal fluid leakage that does occur between the bushing and stem will pass into the internal fluid chamber, whereby, as will be discussed in greater detail below, at least a portion of the corrosive contents of the fluid may precipitate out of the fluid and accumulate in the chamber away from the sealing barriers. Accordingly, the exposure of the sealing barriers to the fluid and its corrosive contents will be minimized to greatly extend the sealing arrangement's worklife. In addition, the use of several sealing barriers will further extend the worklife of the arrangement inasmuch as the fluid will be contained so long as at least one of the barriers remains effective. The series arrangement of the several barriers insures that only one barrier at a time will be eroded away.
In accordance with a significant feature of a preferred embodiment of the invention, the topmost seal of the upper bushing is a pressure activated safety seal. If and when the fluid erodes all of the lower sealing barriers, the fluid pressure will reach the safety seal, whereby the seal will be activated to prevent leakage. Moreover, the safety seal is arranged in a recess formed immediately below the gland follower securing the sealing arrangement. In this manner, the safety seal is easily accessible for replacement. Thus, in the event that the safety seal starts to leak, the valve may be immediately, temporarily repaired by exposing and replacing the safety seal.
Pursuant to a feature of a further embodiment of the invention, the entire seal arrangement is mounted within a removable cartridge. Therefore, the entire sealing assembly of the invention may be conveniently replaced simply by removing a used cartridge from the valve body and inserting a new cartridge in its place. Of course, each cartridge will contain the upper and lower bushings and series of sealing barriers in accordance with the teachings of the present invention.
Thus, the present invention provides a highly effective sealing arrangement for preventing fluid leakage around the valve stem of a high pressure control valve. The novel sealing arrangement of the invention is particularly effective in applications wherein the fluid to be controlled is a high temperature, high pressure fluid containing corrosive material, such as molten salt. The sealing arrangement includes several sealing barriers arranged in an axially spaced series to substantially prolong the amount of time required for the corrosive contents of the fluid to completely corrode and erode the sealing portions of the sealing arrangement. In addition, the invention provides several highly effective features to minimize and impede the exposure of the sealing barrier portions to the high temperature, high pressure fluid, as well as the corrosive contents thereof. In addition, if and when the fluid fully erodes the several sealing barriers, the sealing arrangement of the invention is readily repairable and/or replaceable in a minimal amount of time to greatly enhance and expand the on-line worklife of a high pressure control valve.